As is known, isotactic polypropylene, though being endowed with an exceptional combination of excellent properties, is affected by the drawback of insufficient impact resistance at relatively low temperatures.
According to the teachings of the prior art, it is possible to obviate this drawback, without sensibly affecting the other polymer properties, by modifying the synthesis process or by blending with rubbers.
The modification of the synthesis process comprises, after homopolymerizing propylene to isotactic polymer, copolymerizing in the presence of the isotactic polymer ethylene and propylene mixtures. Processes and compositions representative of the prior art are described in U.S. Pat. Nos. 3,629,368, 3,670,368, 3,670,053 and 3,200,173, and European patent application No. 0077532.
The impact resistance at low temperatures of isotactic polypropylene can be improved by blending with it an ethylene-propylene rubber (see, for example, U.S. Pat. No. 3,627,852).
The products of the modified synthesis process and of blending are characterized by an excellent balance of properties, in particular high rigidity and resistance to impact even at relatively low temperatures. Such products, however, though being particularly suitable for the manufacture of injection molded articles, result in a particularly serious drawback as regards the aesthetic characteristics of articles manufactured therefrom. These products, in fact, are characterized by a low resistance to whitening when subjected to impact. Such phenomenon appears in the form of a whitish spot that forms around the impact-concerned area.
To obviate such drawback, it has been suggested to mix specific propylene-ethylene copolymers, prepared in two steps, with specific types of polyethylene (see for example, U.S. Pat. No. 4,312,964). However, this involves, in addition to the two synthesis process steps, the further step of mixing the copolymers with the polyethylene. This represents a considerable drawback as regards both the economics and the simplicity of producing the end product.